1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to an electrophotographic image forming apparatus, such as a digital color copying machine, comprising means for adjusting image forming means such as charging means, developing means and transfer means, based on a surface voltage of a photoconductor, to stably form an image on a piece of paper with a high precision.
2. Description of the Prior Art
Generally, photoconductive drums used in an electrophotographic process have a dispersion of electrostatic characteristics, and each photoconductive drum has an electrostatic characteristic changing due to change of the environments and/or changing when document images of many pages have been printed out. Moreover, the photoconductive drums may be electrically charged to ununiformly form surface voltages changing in their circumferential direction.
In particularly, in color copying machines, these changes in the electrostatic characteristics may remarkably influence reproducibility of colors, color balance, and reproducibility of a low-density part of an image.
For the purpose of more stably forming images, an automatic density control process is performed which detects electrostatic characteristics of a photoconductive drum and controls a surface voltage thereon, thereby more stably forming images.
The surface voltage of the photoconductive drum is detected by a voltage sensor, and the detection value would involve nonuniformity in the circumferential direction of the photoconductive drum. This nonuniformity may occur because of, for example, deflection due to decentering of the photoconductive drum. As a result, considerable errors may occur in detecting the surface voltage. Thus, attempts to more stably form images should be implemented, taking the voltage nonuniformity into consideration.
An image forming apparatus as described in Japanese Patent Laid-Open Publication No. Sho-62-251763/1987 is so arranged that dark-part and bright-part voltages are detected at predetermined positions of the photoconductive drum which correspond to the average value, the maximum value, and the minimum value of the detected voltages, in order to reduce the effect of nonuniformity in the circumferential voltage of the photoconductive drum.